Logging into the website as someone else while in Admin role.

While I am logged into the website I need to be able to login as some other user and use the web site as that user. When I am done then I want to return to admin status and be able to login as aother user.
This URL helped http://www.4guysfromrolla.com/articles/102208-1.aspx
I can login as another user but I cannot get back to admin status to login as another user.
 
How can I do that?
 
Thanks
 
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